Soil bacteria are known to play an important role in plant growth. For example, some bacteria, especially certain Pseudomonas sp., are able to promote plant growth while other bacteria can promote emergence of plant seedlings. Because the bacteria reside in the soil, their effect on the plant is mediated through colonization in the area of the plant roots, termed the rhizosphere, where agents released by the plant are believed to accumulate to the extent that bacteria are attracted chemotacticly.
Perhaps the most well recognized plant/bacteria relationship is that which results in nitrogen fixation where nitrogen-fixing bacteria infect root hairs of leguminous plants, such as soybean, clover, alfalfa, string beans and peas. The infection leads to nodule formation within which free nitrogen is converted to combined nitrogen (nitrogen-fixation). In this way, legumes are at a selective advantage in unfertilized bare soil but this advantage is dependent upon the presence of appropriate bacteria during plant growth.
Given the beneficial effect which some soil bacteria have on plants, an art has developed which focuses on providing means for introducing selected bacteria to the growth environment of sown seeds or roots of maturing plants.
A variety of such techniques have been proposed in this art. For example, soil within which plants are grown can be directly inoculated with the bacteria. More typically, however, a carrier such as peat moss is impregnated with bacteria which allows the bacteria to survive until the impregnated carrier is placed in the soil. When seed planted with the impregnated carrier germinates and the plant begins to grow, the bacteria present in the inoculated carrier become available for interaction with plants roots e.g. by infecting the root hairs of a legume to establish a symbiotic, nitrogen fixing relationship.
A more promising method proposed in the art, with which the present invention is primarily concerned is that of adhering the bacteria to seed prior to planting. Obviously, where the seed is not to be planted immediately, the bacteria must be capable of surviving on the seed in order to be viable when the seed is planted. Typically, this is accomplished by drying the bacteria on the seed or by adhering a food source to the seed together with the bacteria.
U.S. Pat. No. 4,161,397 to Kalo Laboratories Inc. describes a liquid composition for application to seed in which microdried bacteria are suspended in a liquid carrier i.e. a mineral oil-gel matrix, having a fungicidal agent.
U.S. Pat. No. 4,136,486 describes a seed coating composition in which the bacteria are mixed with water and an adhesive agent such as an alkali metal salt of carboxymethyl cellulose. The composition may be freeze dried for shipping and reconstituted by the end user upon addition of water for application to seeds.
In another prior art proposal, U.S. Pat. No. 4,434,231 discloses a seed-coating composition comprising a polymeric matrix in which bacteria are embedded. The matrix comprises a polymer gel of cross-linked polysaccharide, sources of which include the fermentation products of microorganisms such as Xanthomonas or Arthrobacter or the fungi of the genus Sclerotium. Natural or biosynthetic gums derived from seaweed, plant exudates and seeds may also be used. The polysaccharides are treated prior to formation of the bacterial composition to establish the desired cross-linking such as by heat treatment, treatment by a metal salt or synergism by means of another polysaccharide so as to form a gel for subsequent application to seed once the bacteria are introduced into the gel.